Gravure print surfaces, for instance gravure cylinders, are a common means of supplying liquid compositions to webs. U.S. Pat. No. 4,373,443 describes the use of a gravure cylinder to provide ink in newspaper presses. Engraved upon the surface of the gravure cylinder are cells, which retain the liquid composition after being immersed in the reservoir. A doctor blade scrapes excess liquid composition from the surface of the gravure cylinder, such that the cylinder delivers a precise amount of liquid to a second surface upon contact. A number of distinct feed apparatus types which produce a variety of coating flow patterns are used to coat a gravure cylinder.
One common means of coating a gravure cylinder is a feed pan, similar to U.S. Pat. No. 3,936,549. The pan is constructed to hold a supply of the liquid composition with a means to maintain the liquid level. The gravure cylinder, while partially immersed in the reservoir, rotates to fill the cells and transfer the composition to a web. Drawbacks of the feed pan design include liquid waste due to large holdup volumes and the creation of "flow lines" in the coating. "Flow lines" occur when the gravure cylinder surface initially dips into the liquid composition and contacts a region with impurities. The impurities include air bubbles, globules of concentrated dye and binder, and pieces of partially dried foam. The feed pan design is conducive for the formation of stagnation regions where impurities may form and collect. Experiments with alternative pan designs were unsuccessful in eliminating "flow lines," particularly at high coating speeds.
An additional common means to coat the gravure cylinder is a reverse doctor-pond feed, as seen in U.S. Pat. No. 4,158,333. The pond feed, which may be open or enclosed, is centered at the nine o'clock position with a reverse angled doctor blade at the seven o'clock position. The reverse doctor blade scrapes the surface of the gravure cylinder as the surface exits the reservoir and the doctored cells pass through three quarters of a rotation before reaching the impression nip. The volatile solvents in the liquid composition remaining in the cells evaporate or "dry in" and create a leading edge pattern on the second cylinder or web. Additionally, foam generation on the pond surface may lead to "flow line" problems as with the feed pan method.
Another alternative apparatus to coat the gravure cylinder is an X-hopper coater, which is an extrusion device. A high degree of accuracy and precision are required to position the device relative to the gravure cylinder. The feed applies a thick (2-5 mils) coating to the cylinder at the three o'clock position. The surface remains wetted until reaching the doctor blade at the standard 10 o'clock position. The disadvantage of the design is primarily the cost associated with constructing a precise metering slot and a high pressure metering pump. The design is also inadequate for startup procedures, where the feeder coats the cylinder while the doctor blade and impression nip are disengaged. Under those conditions the excess coating solution would pool and splash at the hopper's lip.
More recent methods developed to coat the gravure cylinder for thermal donor media melts is the feedbar pond, the patent application for which is pending. It combines a knife coater with a puddle or pond feed with a continuous solution delivery means, an overflow and recycle. The knife or feedbar spacing provides a means for metering the coating of solution on the gravure cylinder, as it exits the bottom of the pond, at an three o'clock position, prior to being doctored off at the 10 o'clock position. The main disadvantages of this device is that at high speeds, the pond becomes turbulent and wavy, forming standing oscillating waves and eddies, which can cause `flowline` defects. It also fails to uniformly wet the gravure cylinder at high speeds and viscosity's due to air entrainment at the air-cylinder-solution interface, causing `flowpattern` defects.
The problems identified and solved by the present invention include an apparatus which supplies a uniform curtain-like layer of coating to a gravure cylinder at high coating speeds. More specifically, the apparatus of the invention supplies a coating solution to the gravure cylinder without coating defects associated with high coating speeds such as `flowlines` and `flowpatterns`, and at the same time increases the onset speed of wetting failure due to air entrainment at the cylinder-liquid-air interface. Moreover, the apparatus of the invention reduces waste and costs associated with the gravure coating process.